This invention relates in general to electrophotographic imaging members and more specifically, to electrophotographic imaging members having higher sensitivity.
Multiple layered electrophotographic imaging members comprising photogenerating layers and transport layers deposited on conductive substrates are also well known in the art and are extensively described in the patent literature, for example, in U.S. Pat. No. 4,265,990. These devices usually comprise a substrate having a conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer and, in some embodiments, an anti-curl backing layer.
Although excellent toner images may be obtained with multilayered belt photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, the need for higher sensitivity photoreceptors became greater. Moreover, high sensitivity photoreceptors are desirable for low speed copiers and printers because less power is consumed to expose the photoreceptor. Further, simpler, more compact and less expensive electromagnetic radiation sources may be utilized. Generally, multilayered photoreceptors tend to have a relatively soft photoinduced discharge curve (PIDC). A photoinduced discharge curve obtained by charging a photoreceptor to a given voltage and then exposing to light and plotting the remaining voltage as a function of light exposure. Typically such a curve is characterized by 1) an initial straight portion of the curve associated with sensitivity, S, and 2) the point at which the slope of the initial straight line portion decreases to a value of 1/2 the initial slope, described as the critical voltage V.sub.c and a residual voltage V.sub.r which is the voltage remaining after an excess of light is used to discharge the photoreceptor. Thus, the lower the V.sub.c than the sharper the PIDC. Typically a composite index of curve slope is used which is the ratio of Vc/S. Thus the lower the value of Vc/S, the sharper a curve is said to be. Typically a Vc/S value of approximately 2 or greater would be considered a "soft PIDC". A softer PIDC curve shape indicates that the photoreceptor is less sensitive and requires more light or longer exposure. Thus, a less sensitive photoreceptor will not discharge as much as a more sensitive photoreceptor. Further, insufficient discharge of a photoreceptor results in higher residual charge after a conventional light exposure erase step. High residual charge or potential can be a cause of high background in prints. Thus, there is a continuing need for an electrophotographic imaging member having improved sensitivity.